Zinc hydroxide is a by-product of the treatment of electrogalvanizing wastewater. Steel production is one example of an industrial process that uses electrogalvanizing which produces contaminated wastewater that must be processed to separate out pollutants so that the water can be recycled. During the water treatment process, zinc hydroxide slurries and/or filtercakes are formed. Although zinc hydroxide is a useful industrial compound, as a by-product of the electrogalvanizing process, it is contaminated with iron, manganese and other impurities. One industrial use of zinc hydroxide is as a source of zinc in the production of zinc bromides. Purification is required to remove the various impurities before the zinc or zinc hydroxide can be used to make zinc bromide, or else the impurities must be removed from the zinc bromide. The process of purifying the low-grade zinc hydroxide can be both expensive and time consuming. Using recycled zinc hydroxide containing impurities without the benefit of a purification process, however, results in a contaminated end product.
Keblys, U.S. Pat. No. 4,248,850, discloses a process for preparing basic metal bromides comprising contacting in an aqueous medium a basic metal compound and bromine in the presence of formaldehyde as a reducing agent. The '850 reference teaches the use of alkaline earth metal compounds such as calcium carbonate, lime or zinc hydroxide. The example methods disclosed in the '850 patent reference resulted in a pH range of above 5.7. The '850 reference does not teach or suggest how to remove the iron and/or manganese impurities from zinc hydroxide.
A process for producing metal bromides is disclosed in Sanders, U.S. Pat. No. 4,083,942. Alkali and alkaline earth metal bromides may be prepared by reacting a basic compound of an alkali or alkaline earth metal with a reducing agent, formic acid, to form a reaction mixture at a pH of less than 7.0; and thereafter, adding stepwise alternate incremental portions of bromine and the basic compound while maintaining the pH less than 7, preferably using lime as the basic compound.
Beaver et al., U.S. Pat. No. 5,230,879 teaches a process for reducing metal halates to metal halides by reacting a metal halate with a reducing agent in an aqueous alkaline reaction medium having a pH above about 7. Kirsch U.S. Pat. No. 4,514,374 discloses the preparation of metal bromides from alkaline earth metal compounds in the presence of added lower alkanol, preferably methanol, as a reducing agent.
None of the references teach or suggest using zinc hydroxide recovered from electrogalvanizing wastewater that is contaminated with iron and/or manganese to manufacture high purity zinc bromide, nor a method for doing so.
The problem of producing zinc bromine from zinc hydroxide slurries or filtercakes that are byproducts of industrial processes, such as electrogalvanizing wastewater, is that the resulting zinc bromine is contaminated with impurities that prevent it from being marketable. This problem has not been adequately resolved by prior teachings. It is desirable to have a method of producing zinc bromine from contaminated zinc hydroxide that removes iron, manganese and other metal impurities without the release of excess bromine gas.